This invention relates to a process for the preparation of anhydrous sodium sulfate and, more specifically, to a process for the benefitiation of thenardite ores to produce anhydrous sodium sulfate.
The known conventional process for the production of sodium sulfate, begins with from a brine which is extracted from the underground and stored in a reservoir and from there it is conducted to a crystallizer wherein, by cooling, crystals are produced in the form of a fine powder of sodium sulfate hydrated with ten molecules of water, known as Glauber salt, which is settled, then filtered and conducted to a reservoir wherein it is melted to dissolve the crystals of sodium sulfate in its own hydrating water, to produce a saturated solution of sodium sulfate with 17% of solids, which is then partially evaporated to obtain a suspension of anhydrous sodium sulfate with 20% of solids in a saturated solution, said suspension is then settled and centrifuged to concentrate the solids of anhydrous sodium sulfate which are separated and dried recycling the remaining saturated solution back to the evaporation step.
In said conventional process the crystallization of the Glauber salt is carried out by cooling it at about 20.degree. C. to about 0.degree. C. in a special crystallizer, including additional equipment such as boilers, cooling towers, boosters, etc.
The evaporation step has been considered as strictly necessary to concentrate the solids of anhydrous sodium sulfate up to 20%.
It is well known that there are thenardite ores in several deposits around the world in different concentrations, some of which have a higher content of sodium sulfate than that of the brine deposits.
However, one of the problems associated with the production of anhydrous sodium sulfate from an ore such as the thenardite is the quality (fineness). It is well known that the anhydrous sodium sulfate obtained by the known processes, contain small quantities (less than 1%) of fine clay, that product a dark aspect in the solution.
Another disadvantage of the known processes to produce anhydrous sodium sulfate is the investment in equipment that is very expensive and sophisticated.
With this backgrounds, the inventor began his research with respect to a deposit of thenardite ore, located in Bacadehuachi in the Mexican state of Sonora to extract anhydrous sodium sulfate from the ore to be used mainly in the production of detergents, paper and glass.
Based on the curve of solubility for sodium sulfate, the inventor found in his research that it was possible to obtain a 32% saturated solution from the ore and that it was necessary to lixiviate the ore with water at a temperature of 32.degree. C., because at a higher temperature, a decay in the solubility was achieved.
At a temperature below 32.degree. C., the Glauber salt crystallizes and, in the process of the present invention, such crystallization is carried out in an open reservoir with considerable savings in energy because of the ambient conditions of the place of the deposit wherein the ambient temperature changes from 30.degree. C. or 40.degree. C., to 10.degree. C. or 20.degree. C. at night, which cooling helps in the natural crystallization of the Glauber salt.
Moreover, the inventor found that it is important to crystallize the solution in a reservoir, wherein it is cooled overnight at a temperature between about 10.degree. C. to about 20.degree. C., taking care that the height of the solution within the reservoir does not exceed approximately 15 cm., to produce the Glauber salt in the form of large crystals which allows its separation by simply draining.
This Glauber salt is then melted at a temperature between about 40.degree. C. and 60.degree. C. and is dissolved in its own crystallization water to crystallize the anhydrous sodium sulfate which is separated and dried.
It is also important that the saturated solution resulting from the melting of the Glauber salt, by recycled to the crystallization step instead of evaporating it, as in the conventional brine process, to avoid the need for evaporation equipment and the energy consumption involved.